Conventionally powder for dispersion nuclear fuel is produced by alloying and comminution. Alloying metals are alloyed into ingots by induction or arc heating in a vacuum atmosphere. The as-cast ingots are heat-treated in a vacuum for 100 hours at 900.degree. C. to ensure compositional homogeneity, and then quenched to form a meta-stable gamma phase. The ingots are machined into chips and milled under liquid argon using a hardened steel mill to obtain the appropriate particle size. The chips of uranium alloys are very pyrophoric due to its high oxidative characteristics. Thus, it is necessary to machine under a sufficient amount of cutting fluid to substantially prevent oxidation. The fuel powder is contaminated by the cutting fluid. Processes of rinsing with an organic solvent such as acetone etc. and drying under vacuum atmosphere at a high temperature are required. Also, during milling the small particles containing ferrous impurities are introduced by the wear of milling machine parts. A close-up of the particle surface reveals many dark spots on the surface which energy dispersive spectroscopy has determined to be iron-rich. Most of the particles containing ferrous components are removed by magnetic separation.
As it is difficult to comminute uranium alloy ingots due to its tough property, the yield of uranium alloy through a mechanical powdering process, which consists of many steps of chipping, milling, rinsing, and drying, is very low, in the range of 5 to 20%. In addition, during magnetic separation about 30% of fuel powder is lost because separated powder contains a considerable amount of fuel particles.
In the case of directly making powder having a particle size from alloy ingots using a high speed lathe equipped with a rotary file, the productivity of usable powder is very low, as it yields 12 grams per hour. The yield of powder smaller than 212 .mu.m ranges from 32.about.63% of the total powder, depending on the alloy composition. The powder is produced by grinding ingots with a tungsten/tantalum carbide tool rotating at approximately 2,500 rpm. This process has the drawback of carbide and nitride contamination in the powder due to the wear of the rotary file. Contamination levels range from 0.1.about.7.6% and are generally higher for uranium alloys with larger alloy contents.
The comminuted particles with longish and irregular shapes, arranged along the rolling or extruding direction perpendicular to heat flow, inhibit thermal conduction in fuel meat. The large specific surface area of these irregular particles enhances the interaction between the fuel particles and an Al matrix to form uranium-aluminide (UAl.sub.x) with low-density around the perimeter of the uranium alloy particles, with the consequence of thermal swelling of the nuclear fuel meats.